Ball screw device and linear motion device

ABSTRACT

In a ball screw device comprising a screw shaft and a nut which make relative movement to each other through a multiplicity of balls, as well as in a linear motion device comprising an outer member and an inner member which make the relative movements to each other through the multiplicity of balls, a spacer having two concave surfaces facing respectively to balls, is disposed between the balls adjacent to each other. A section of each of the concave surfaces of the spacer is formed of two circular arcs of which central positions deviate from each other to form a Gothic arch. The spacer has such a configuration that the balls adjacent to each other come into contact with outer edges thereof or portions vicinal to the outer edges. The spacer also has such a configuration that the balls adjacent to each other come into contact with at least three or more outer edge portions thereof or portions vicinal to the outer edges. The spacer may have a through-hole formed in a thinnest portion thereof.

[0001] This application claims the benefits of Japanese Application Nos.10-167813, 10-239458 and 11-026544 which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a ball screw device and a linearmotion device which do not induce decreases in load capacity and inrigidity with a restraint of reduction in the number of load balls evenwhen spacers are disposed between the load balls, which enhance acirculative characteristic of the spacers by minimizing friction betweenthe load balls and the spacers, and which prevent a deterioration ofoperability, an occurrence of noises and deteriorated quality of a soundproduced, and frictional damage to the balls.

[0004] 2. Related Background Art

[0005] In the ball screw device, as shown in FIG. 33, helical screwgrooves 3, 4 corresponding to each other are formed in an outerperipheral surface of a screw shaft 1 and in an inner peripheral surfaceof a nut 2. A multiplicity of balls 5 are so disposed as to be capableof rolling in a helical circulation path defined by the two screwgrooves 3, 4. When one of the screw shaft 1 and the nut 2 is moved inthe axial direction by relatively rotating the screw shaft 1 and the nut2, the screw shaft 1 and the nut 2 make smooth helical motions throughrolling of the multiplicity of balls 5.

[0006] In the thus constructed ball screw device, the balls 5 aredensely disposed within the screw grooves 3, 4 and roll in the samedirection in the individual screw grooves 3, 4. On this occasion,however, at a contact point between the balls adjacent to each other,the balls 5 rolling in the directions opposite to each other come intocontact with each other enough to mutually hinder the rolling thereof.As a result, there might arise a variety of problems in which freerolling of the balls 5 is thus hindered, an operability of the balls 5is deteriorated, frictional damage to the balls 5 is induced, a torquefluctuates, and noises increase.

[0007] To cope with these problems, Japanese Patent ApplicationLaid-Open Publication No.56-116951 discloses a construction wherein anelastic member for making the balls spaced away from each other isdisposed between the balls receiving a load, and an annular membermaking a circulative movement together with the balls is loosely fittedto the outside of the elastic member. Japanese Patent ApplicationLaid-Open Publication No.57-101158 discloses such a construction that ashim is retained between the adjacent balls and serves to prevent arolling friction between the balls.

[0008] Further, Japanese Utility Model Application Laid-Open PublicationNo.1-113657 discloses a construction in which a spacer ball 6 formedfrom a resin is, as illustrated in FIG. 34, interposed between the balls5 receiving the load, thereby preventing the impingement of the balls oneach other and restraining an occurrence of noises.

[0009] Incidentally, what is similar to the ball screw device describedabove may be exemplified by a linear guide constructed of a guide railextending in the axial direction, a slider provided astride of thisguide rail, and balls serving as rolling members and interposed betweenthe guide rail and the slider. The above-mentioned ball screw device andlinear guide are generically referred to as a linear motion device inthe present specification. The linear motion device is defined as beingconstructed of an outer member, an inner member facing to this outermember through a gap, a multiplicity of balls disposed between the outerand inner members, and spacers interposed between those balls.

[0010] For example, in the case of the linear guide, the slider havingsubstantially U-shape section is mounted astride of the guide railhaving an angular bar-like shape, track grooves are formed respectivelyin an outer surface of the guide rail and in an inner surface of theslider which faces thereto, and the multiplicity of balls as the rollingmembers are loaded in the track grooves, whereby the slider and theguide rail make relative linear motions with the aid of the rollingmembers circulated while rolling. In the case of this type of linearguide, the slider is defined as the outer member, while the guide railis defined as the inner member. On the other hand, another type oflinear guide has such a construction that an angular slider isaccommodated in a recessed portion of the guide rail takingsubstantially the U-shape in section, and the balls are loaded in thetrack grooves formed respectively in the inner surface of the guide railand in the outer surface of the slider which faces thereto. In thiscase, the guide member is defined as the outer member, while the slideris inner member.

[0011] Further, in the ball screw device, as described above, the screwshaft, of which the outer surface is formed with the helical screwgroove, is inserted into the nut with its inner surface formed with thehelical screw groove, and the multiplicity of balls are loaded in thetwo screw grooves facing to each other. With these balls making therolling circulation, the nut and the screw shaft perform their relativerotational and linear motions. Accordingly, in the case of the ballscrew device, the nut is defined as the outer member, while the screwshaft is defined as the inner member.

[0012] To summarize, the outer member of the linear motion deviceindicates the slider or the guide rail in the case of the linear guide,and indicates the nut in the case of the ball screw device. The innermember indicates the guide rail or the slider in the case of the linearguide, and indicates the screw shaft in the case of the ball screwdevice.

[0013] An example of the above linear motion device using a spacer isdisclosed in Japanese Patent Application Laid-Open PublicationNo.5-126148, wherein as shown in FIG. 35 a spacer 7 having two concavesurfaces 6, 6 contiguous respectively to balls 5, 5 is disposed betweenthe balls 5, 5 adjacent to each other. Further, as for a bearing,Japanese Patent Application Laid-Open Publication No.62-118116 disclosesa-structure that as shown in FIG. 36 a hollowed pipe-like spacer 8 isdisposed between the adjacent balls 5, 5. The spacer 8 is formed bycutting off to a predetermined dimension a steel pipe of which adiameter is smaller than a diameter of the ball 5. Further, as disclosedin Japanese Patent Application Post-Exam Publication No.40-24405, apartition member disposed between the adjacent balls has two sphericalconcave portions each facing to a ball, a radius of which is slightlylarger than a radius of the ball. A through-hole formed at the center ofthe spherical concave portion of the partition member is used as areservoir of lubricating oil.

[0014] A problem inherent in only the ball screw device described aboveis that the spacer, such as the elastic member, the annular member andthe shim etc, is provided in each of the ball screw devices disclosed inJapanese Patent Application Laid-Open Publication Nos.56-116951 and57-101158, and therefore the number of the balls receiving the load isreduced, with the result that a load capacity and a rigidity of the ballscrew device decrease.

[0015] Additionally, the spacer, such as the elastic member, the annularmember and the shim etc, induces an impingement upon the screw grooveenough to cause a skew (from a proper posture) of the spacer, resultingin a decline of a circulative characteristic of the spacer.

[0016] In the ball screw device disclosed in Japanese Utility ModelApplication Laid-Open Publication No.1-113657, as shown in FIG. 34, thenumber of the balls receiving the load is, e.g., 10, while the number ofthe spacers 6 is, e.g., 10, whereby a spacing between the balls 5receiving the load becomes large, the number of the balls 5 receivingthe load is approximately halved, and both of the load capacity and therigidity of the ball screw device decrease.

[0017] Another problem with respect to the linear motion device in theprior art explained above is that it is desirable to make a slidefriction between the spacer and the ball as small as possible in termsof considering an operability of the linear motion device. However, asshown in FIG. 35, if a curvature (1/r) of the ball 5 is equalized to acurvature (1/R) of the spacer concave surface 6, sliding occurs when theball comes into contact with the entire concave surface of the spacer,with the result that the frictional force increases and the operabilityis deteriorated.

[0018] It is very important in this linear motion device to control athickness of the spacer in order to set an optimum total gap in eachtrain of balls endlessly circulated, i.e., to control an inter-ball spanwhen the spacer is interposed therebetween. But

[0019] when manufacturing the spacer 7 aiming at forming the concavesurface 6 having the same curvature (1/R) as the curvature (1/r) of theball 5,

[0020] there might be formed the concave surfaces 6 having larger andsmaller curvatures than the curvature (1/r) of the ball 5 because of adimensional scatter.

[0021] Especially if the curvature (1/R) of the concave surface 6 of thespacer 7 is smaller than the curvature (1/r) of the ball 5, the ballsare destabilized when the spacer 7 is disposed between the balls 5, andit is extremely difficult to measure a dimension between the balls 5(which is a thickness of the spacer 7). The problem is therefore thatthe spacer 7 exhibiting a high accuracy can not be manufactured.Moreover, in a structure as shown in FIG. 36, it is required that thediameter of the spacer be smaller than the diameter of the ball.However, as shown in FIG. 36, in the case of the pipe-like spacer 8, aminor diameter of the pipe-like spacer 8 becomes small due to thethickness thereof, and the balls 5 are hard to stabilize. There is noalternative but to increase the major diameter of the pipe-like spacer 8for stabilizing the balls 5.

[0022] Consequently, there arises a problem in which the spacer 8interferes with other components during the circulation.

[0023] According to Japanese Patent Application Post-Exam PublicationNo.40-24405, the through-hole formed in the partition member is used asthe reservoir of the lubricating oil for preventing a seizure if arotating velocity and a revolution velocity of the ball are high as inthe case of a rolling bearing. In the linear motion device, however,almost no seizure problem arises because of the above velocities beingby far lower than those of the rolling bearing. A further problem in theprior art example is that a lubricating oil reserving capacity of thethrough-hole is insufficient.

SUMMARY OF THE INVENTION

[0024] It is a primary object of the present invention, which wasdevised under such circumstances, to provide a linear motion device(e.g., ball screw device, linear guide device) which is capable ofavoiding decreases in load capacity and rigidity with a restraint ofreducing the number of load balls even when spacers are disposed betweenthe load balls, which enhances a circulative characteristic of thespacer by minimizing friction between the load balls and the spacer, andwhich prevents deterioration of an operability and an occurrence ofnoises due to impingement between the balls, a deteriorated quality ofsound produced, and frictional damage to the balls.

[0025] To accomplish the above object, according to a first aspect ofthe present invention, a ball screw device comprises a screw shaft ofwhich an outer peripheral surface is formed with a helical screw groove,a nut of which an inner peripheral surface is formed with a helicalscrew groove corresponding to the helical screw groove of the screwshaft, a helical circulation path defined by the two helical screwgrooves, and a multiplicity of balls so disposed in the helicalcirculation path as to be capable of rolling. A spacer having twoconcave surfaces facing respectively to the balls is disposed betweenthe balls adjacent to each other, and a section of each of the concavesurfaces of the spacer is formed of two circular arcs of which centralpositions deviate from each other to form a Gothic arch.

[0026] According to the first aspect of the present invention, thespacer having the two concave surfaces facing adjacent balls, isdisposed between the adjacent balls. The spacer takes such aconfiguration of the concave surface that the adjacent balls come intolinear- or point-contact with the concave surface with a smaller slideresistance. For instance, the section of each concave surface of thespacer is formed of two circular arcs of which the central positionsdeviate from each other to form a Gothic arch. Therefore, the load ballscan be well circulated through within the helical screw grooves whilecontacting the spacer concave surfaces.

[0027] The ball screw device is therefore capable of reducing thefriction between the load balls and the spacers, enhancing thecirculative characteristic of the spacer, and preventing thedeterioration of the operability and the occurrence of noises due to theimpingement of the balls on each other, the deteriorated quality ofsound produced, and the frictional damage to the balls. The spacer hassuch a configuration that a thickness thereof is smaller than that ofthe spacer ball, and hence there is no possibility of inducing thedecreases in load capacity and in rigidity with the restraint ofreducing the number of the load balls.

[0028] In the ball screw device according to the first aspect of theinvention, supposing that all the balls and all the spacers insertedinto the helical circulation path be converged on one side, a gap formedbetween a leading ball and a tailing spacer is termed a total gap, andgiven that a spacing (S1) of this total gap is larger than zero (S1>0)and that the one spacer, i.e., the tailing spacer be eliminated, thenumber of the balls and the number of the spacers are set so that aspacing (S2) of a gap between the leading ball and a tailing ball issmaller than a 0.8-fold value of a diameter (ds) of the spacer(S2<0.8×ds).

[0029] As described above, the total gap in the circulation path is setlarger than zero, and one spacer is eliminated, at which time thespacing of the gap between the leading ball and the tailing ball is setin the relationship of the numerical values given above. In this case,it never happens that the spacer is skewed within the circulation pathbecause of the gap in the circulation path being too large. It too neverhappens that an operational defect is caused by the friction between theballs and the spacer because of the gap in the circulation path beingtoo small. The intra-circulation-path gap is properly set, and thereforethe spacer is not skewed at approximately 60° or greater, and goodoperability can be maintained.

[0030] In the ball screw device according to the first aspect of thepresent invention, it is preferable that the spacer be so constructed asto be elastically deformable between the adjacent balls.

[0031] The spacer is thus so constructed as to be elastically deformablebetween the adjacent balls, in which case a ball-to-ball distance can becontrolled through the elastic deformation of the spacer. Accordingly, acharging rate of the balls and the spacer with respect to a circuitlength can be extremely easily set to a proper value. For example, thecharging rate can be controlled by one type of spacers, which obviates atroublesome design work of preparing several types of spacers on a trialbasis and combining these spacers. Further, it is also possible toattain a charging rate of 100% (i.e., the spacing between the ball andthe spacer is zero) as the necessity may arise. Note that the spacer maybe elastically deformed in terms of a structure, or may also beelastically deformed based on only the material itself.

[0032] According to a second aspect of the present invention, a ballscrew device comprises a screw shaft of which an outer peripheralsurface is formed with a helical screw groove, a nut of which an innerperipheral surface is formed with a helical screw groove correspondingto the helical screw groove of the screw shaft, a helical circulationpath defined by the two helical screw grooves, and a multiplicity ofballs so disposed in the helical circulation path as to be capable ofrolling. In this ball screw device, a spacer having two concave surfacesfacing adjacent balls is disposed between the balls adjacent to eachother, and supposing that all the balls and all the spacers insertedinto the helical circulation path be converged on one side, a gap formedbetween a leading ball and a tailing spacer is termed a total gap, andgiven that a spacing (S1) of this total gap is larger than zero (S1>0)and that the one spacer, i.e., the tailing spacer be eliminated, thenumber of the balls and the number of the spacers are set so that aspacing (S2) of a gap between the leading ball and a tailing ball issmaller than a 0.8-fold value of a diameter (ds) of the spacer(S2<0.8×ds).

[0033] As explained above, the total gap in the circulation path is setlarger than zero, and one spacer is eliminated, at which time thespacing of the gap between the leading ball and the tailing ball is setin the relationship of the numerical values given above. Hence, it neverhappens that the spacer is skewed within the circulation path because ofthe gap in the circulation path being too large. It too never happensthat an operational defect is caused by the friction between the ballsand the spacer because of the gap in the circulation path being toosmall. The intra-circulation-path gap is properly set, and therefore thespacer is not skewed at approximately 60° or greater, and goodoperability can be maintained.

[0034] According to a third aspect of the present invention, a linearmotion device comprises an outer member, an inner member facing to theouter member via a gap, a multiplicity of balls disposed between theouter member and the inner member, and a spacer interposed between theballs. In this linear motion device, the spacer has such a configurationthat the balls adjacent to each other come into contact with outer edgesthereof or portions vicinal to the outer edges.

[0035] Thus, in the linear motion device according to the third aspectof the present invention, the spacer has such a configuration that theadjacent balls come into contact with the outer edges or the portionsvicinal to the outer edges. Accordingly, the spacer is capable ofretaining the ball in a much wider area, and it is feasible to take astill larger retaining allowance for the spacer to retain the ball.Furthermore, the ball is easy to stabilize, and a measurement of adimension (i.e., a thickness of the spacer) between the balls isfacilitated, whereby the spacer exhibiting a high precision can bemanufactured.

[0036] According to a fourth aspect of the present invention, a linearmotion device comprises an outer member, an inner member facing to theouter member via a gap, a multiplicity of balls disposed between theouter member and the inner member, and a spacer interposed between theballs. In this linear motion device, the spacer has concave surfaceswith which the balls adjacent to each other come into linear contact.

[0037] Thus, in the linear motion device according to the fourth aspectof the present invention, the spacer is interposed between the balls andhas the concave surfaces with which the adjacent balls come into linearcontact. Accordingly, the friction between the balls and the spacer issmall, and it is feasible to prevent the decline of the operability andthe occurrence of noises due to the impingement of the balls on eachother, the deteriorated quality of sound produced, and the frictionaldamage to the ball.

[0038] In the linear motion device according to the third or fourthaspect of the present invention, the spacer has such a configurationthat the adjacent balls are brought into contact with at least three ormore portions of the spacer.

[0039] As described above, the spacer assumes the configuration that theadjacent balls come into contact with at least three or more portions ofthe spacer, in which case, the balls can contact the spacer with anextremely small friction. The friction between the balls and the spacercan be remarkably reduced by decreasing a slide resistance between theballs and the spacer, and the circulative characteristic of the ballsand the spacers is enhanced. At the same time, the balls are easy tostabilize, and a lubricating agent can be easily led to the spacer. Theslide resistance between the balls and the spacer can be made farsmaller.

[0040] According to a fifth aspect of the present invention, a linearmotion device comprises an outer member, an inner member facing to theouter member via a gap, a multiplicity of balls disposed between theouter member and the inner member, and a spacer interposed between theballs,

[0041] wherein the spacer has a through-hole formed in a thinnestportion thereof.

[0042] As explained above, according to the fifth aspect of the presentinvention, the spacer has the through-hole formed in the thinnestportion thereof. In the linear motion device, a rotating velocity and arevolution velocity of the ball are very low as compared with a rollingbearing, and therefore almost no seizure problem arises. A contact areabetween the balls and the spacer becomes, however, far smaller owing tothe through-hole of the spacer, and a fluctuation in kinetic frictionforce can be made extremely small. At the same time, there is anadvantage that an influence upon a strength thereof is remarkably smallbecause of the through-hole being formed in the minimum-thicknessportion between the concave surfaces.

[0043] Other features and advantages of the present invention willbecome readily apparent from the following description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles-of the invention, in which:

[0045]FIG. 1A is a side view showing a ball screw device in a firstembodiment of the present invention; FIG. 1B is a sectional view showinga spacer installed in the ball screw device shown in FIG. 1A;

[0046]FIG. 2A is an enlarged view showing balls and the spacer of theball screw device shown in FIGS. 1A and 1B; FIG. 2B is an explanatoryview showing a configuration of Gothic arch;

[0047]FIG. 3 is an enlarged view of the spacer of the ball screw deviceshown in FIG. 1B as viewed in a direction C in FIG. 1B;

[0048]FIG. 4A is a partial side view showing the ball screw device in afirst modification of the first embodiment of the present invention;FIG. 4B is an explanatory view showing the principle of the firstmodification;

[0049]FIG. 5 is a partial side view showing the ball screw device in asecond modification of the first embodiment of the present invention;

[0050]FIG. 6 is a partial side view showing the ball screw device in athird modification of the first embodiment of the present invention;

[0051]FIG. 7 is a plan view showing the ball screw device in a fourthmodification of the first embodiment of the present invention;

[0052]FIG. 8A is an explanatory view showing the principle of the ballscrew device in a second embodiment of the present invention; FIG. 8B isa sectional view of the spacer;

[0053]FIG. 9 is a side view showing the ball screw device in the secondembodiment of the present invention;

[0054]FIG. 10 is a side view showing the ball screw device in amodification of the second embodiment of the present invention;

[0055]FIG. 11 is an enlarged view showing the balls and the spacer ofthe ball screw device in a third embodiment of the present invention;

[0056]FIG. 12 is a side view showing the ball screw device in the thirdembodiment of the present invention;

[0057]FIG. 13 is an enlarged view showing the balls and the spacer ofthe ball screw device in a modification of the third embodiment of thepresent invention;

[0058]FIG. 14 is a perspective view showing a linear guide in a fourthembodiment of the present invention;

[0059]FIG. 15 is an enlarged sectional view of the linear guide shown inFIG. 14;

[0060]FIG. 16 is an enlarged sectional view showing the balls installedin the linear guide shown in FIG. 14, and the spacer interposed betweenthe balls;

[0061]FIG. 17 is an enlarged sectional view showing the balls installedin the linear guide in a first modification of the fourth embodiment ofthe present invention, and the spacer interposed between the balls;

[0062]FIG. 18 is an enlarged sectional view showing the balls installedin the linear guide in a second modification of the fourth embodiment ofthe present invention, and the spacer interposed between the balls;

[0063]FIG. 19 is an enlarged sectional view showing the balls installedin the linear guide in a third modification of the fourth embodiment ofthe present invention, and the spacer interposed between the balls;

[0064]FIG. 20 is an enlarged sectional view showing the balls installedin the linear guide in a fourth modification of the fourth embodiment ofthe present invention, and the spacer interposed between the balls;

[0065]FIG. 21 is an enlarged sectional view showing the balls installedin the linear guide in a fifth modification of the fourth embodiment ofthe present invention, and the spacer interposed between the balls;

[0066]FIG. 22 is an enlarged sectional view showing the balls installedin the linear guide in a sixth modification of the fourth embodiment ofthe present invention, and the spacer interposed between the balls;

[0067]FIG. 23 is an enlarged sectional view showing the balls installedin the linear guide in a seventh modification of the fourth embodimentof the present invention, and the spacer interposed between the balls;

[0068]FIG. 24A is a sectional view showing the spacer installed in thelinear guide in a fifth embodiment of the present invention; FIG. 24B isa side view of the spacer shown in FIG. 24A;

[0069]FIG. 25 is a sectional view showing the spacer installed in thelinear guide in the sixth embodiment of the present invention;

[0070]FIG. 26 is a sectional view showing the spacer installed in thelinear guide in a modification of the sixth embodiment of the presentinvention;

[0071]FIG. 27 is a graph showing a result of a test in an example of thesecond embodiment of the present invention;

[0072]FIG. 28 is a graph showing a result of the test in a comparativeexample 1 of the second embodiment of the present invention;

[0073]FIG. 29 is a graph showing a result of the test in a comparativeexample 2 of the second embodiment of the present invention;

[0074]FIG. 30 is a graph showing a result of the test in a comparativeexample 3 of the second embodiment of the present invention;

[0075]FIG. 31 is a graph showing a result of the test in an example ofthe sixth embodiment of the present invention;

[0076]FIG. 32 is a graph showing a result of the test in a comparativeexample of the sixth embodiment of the present invention;

[0077]FIG. 33 is a side view showing a ball screw device in the priorart;

[0078]FIG. 34 is a side view showing another ball screw device in theprior art;

[0079]FIG. 35 is a sectional view showing the balls and the spacer inthe prior art; and

[0080]FIG. 36 is a sectional view showing other balls and spacer in theprior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0081] A ball screw device and a linear motion device in preferredembodiments of the present invention will hereinafter be described withreference to the accompanying drawings.

[0082] First through third embodiments deal with the ball screw device,and fourth through sixth embodiments deal with a linear guide.

[0083] (First Embodiment)

[0084]FIG. 1A is a side view showing a ball screw device in a firstembodiment of the present invention. FIG. 1B is a sectional enlargedview showing a spacer installed in the ball screw device illustrated inFIG. 1A. FIG. 2A is an enlarged view showing a ball of the ball screwdevice shown in FIG. 1A and the spacer. FIG. 2B is an explanatory viewshowing a configuration of Gothic arch. FIG. 3 is an enlarged viewshowing the spacer of the ball screw device illustrated in FIGS. 1A and1B.

[0085] As illustrated in FIG. 1A, helical screw grooves 3, 4corresponding to each other are formed in an outer peripheral surface ofa screw shaft 1 and in an inner peripheral surface of a nut 2. Amultiplicity of balls 5 are so disposed as to be rollable within ahelical circulation path defined by the two screw grooves 3, 4. When oneof the screw shaft 1 and the nut 2 is moved in an axial direction byrelatively rotating the screw shaft 1 and the nut 2, the screw shaft 1and the nut 2 make a smooth relative helical motion through rolling ofthe multiplicity of balls 5. Note that a ball circulating method in theball screw device in the first embodiment is applicable to all typessuch as a circulation piece type, an end cap type and a tube type etc.

[0086] A multiplicity of spacers 10 each made from a sphere areinterposed between the neighboring balls 5 receiving a load. The spacer10 is, as shown in FIG. 1B, formed with two concave surfaces 11, 11.

[0087] A section of each concave surface 11 is formed of two circulararcs of which central positions deviate from each other to form a Gothicarch. To be more specific, the Gothic arch assumes, as shown in FIG. 2B,such a configuration that the two central positions each having a radiusR deviate a predetermined distance from each other. As illustrated inFIG. 2A, the two central positions (X, X) of each concave surface 11,with lines connecting the respective circular arcs to the centers X,i.e., radiuses intersecting each other at a central position (Y) of theball 5, deviate the predetermined distance from each other.

[0088] Thus, the section of each concave surface 11 takes the Gothicarch shape, and therefore, as shown in FIG. 3, the ball 5 is allowed tolinearly contact the concave surface 11 of the spacer 10 in a circularshape shown by a broken line Z.

[0089] Accordingly, the ball 5 is capable of being brought into contactwith the concave surface 11 of the spacer 10 with an extremely smallfriction. And it is feasible to remarkably reduce the frictiontherebetween by decreasing a slide resistance between the ball 5 and thespacer 10. Hence, a circulative characteristic of the spacer 10 isenhanced, and it is also possible to highly restrain the balls 5 frombeing deteriorated in terms of their operability due to impingementbetween these balls 5 and from being damaged due to the frictionstherebetween. It too never happens that fluctuations in torque and anoise problem might be induced.

[0090] The spacer 10 can be made far smaller in configuration than theprior art spacer ball, and hence the number of the balls 5 receiving theload can be significantly higher than in the conventional art. That is,in the conventional ball screw device having the spacer balls shown inFIG. 34 the number of the load supporting balls 5 is 10, and the numberof the spacer balls 6 is 10. By contrast, in the ball screw device inthe first embodiment shown in FIG. 1A, the number of the load supportingballs 5 is 18, and the number of the spacers 10 is 18. The number of theload supporting balls 5 is approximately doubled as compared with theprior art. Accordingly, the invention does not exhibit the problem thata load capacity or a rigidity might decrease due to reduction in thenumber of the load balls 5.

[0091] Note a ratio of the number of the balls 5 to the number of thespacers 10 is 1:1 in the example shown in FIG. 1A and may be, as amatter of course, 2:1 or 3:1.

[0092]FIG. 4A is a partial side view of the ball screw ball device,showing a first modification of the first embodiment of the presentinvention. FIG. 4B is an explanatory view showing the principle of thefirst modification.

[0093] If a diameter of the sphere for constructing the spacer 10 isequalized to a diameter of the ball 5, as shown in FIG. 4B, when theball 5 is so disposed as to contact the concave surface 11 of the spacer10, it follows that the spacer 10 comes to an interference with thescrew groove 3.

[0094] Accordingly, in the first modification of the first embodiment ofthe present invention, as shown in FIG. 4A, a center C between centralpositions (Y, Y) of the two balls 5, 5 adjacent to each other is set asa center of the sphere for constructing the spacer 10, and a diameter(d) of the sphere is set such that a radius is a distance or shorterfrom the central point c to the screw groove 3. Therefore, it neverhappens that the spacer 10 is in the interference with the screw groove3, and the spacer 10 with a reduced diameter can be disposed withstability between the balls 5 to achieve a good operability.

[0095]FIG. 5 is a partial side view of the ball screw device, showing asecond modification of the first embodiment of the present invention.

[0096] In the second modification of the first embodiment, the spacer 10is formed with a through-hole 12 between the two concave surfaces 11,11. The through-hole 12 contains a lubricating agent such as lubricatinggrease and oleo-resin etc. With this lubricating agent, the slideresistance between the ball 5 and the spacer 10 is further decreased,and the friction therebetween can be remarkably reduced, whereby thecirculative characteristic of the spacer 10 can be more enhanced. Notethat the ball circulating method in the ball screw device in the secondmodification of the first embodiment is applicable to all types such asthe circulation piece type, the end cap type and the tube type etc.Further, a retainability with respect to the through-hole 12 is alsoenhanced by use of the grease and the oleo-resin.

[0097]FIG. 6 is a partial side view of the ball screw device, showing athird modification of the first embodiment of the present invention.

[0098] In the third modification of the first embodiment, the spacer 10is formed with the through-hole 12 between the two concave surfaces 11,11. A small-diameter ball 13 is disposed in this through-hole 12.

[0099] The small-diameter ball 13 is brought into rolling-contact withthe ball 5, while the spacer 10 comes into (not linear contact but)point-contact with the ball 5. It is therefore possible to furtherreduce the slide resistance between the ball 5 and the spacer 10, and tothus remarkably decrease the friction therebetween. The circulativecharacteristic of the spacer 10 can also be enhanced.

[0100] Note that the ball circulating method in the ball screw device inthe third modification of the first embodiment is applicable to alltypes such as the circulation piece type, the end cap type and the tubetype etc.

[0101]FIG. 7 is a plan view of the ball screw device, showing a fourthmodification of the first embodiment of the present invention.

[0102] The ball screw device in the fourth modification of the firstembodiment is classified as a tube circulation type ball screw deviceincluding a circulation tube 14, formed in association with the screwgrooves 3, 4, through which to circulate the balls 5 and the spacers 10.

[0103] The circulation tube 14 is also formed with a bend radius.According to the fourth modification of the first embodiment, this bendradius (R) is set equal to a radius of a ball center diameter (BCD) ofthe screw grove 3 of the screw shaft 1. Thereby, the spacer 10 made fromthe sphere having the diameter (d) as set in the first modification ofthe first embodiment, is capable of passing through the circulating tube14 having the bend radius with a good operability.

[0104] It is to be noted that the first embodiment of the presentinvention can be modified in many ways. For example, a material fromwhich the spacer 10 is formed may be a steel, the oleo-resin, a resin oran oleo-resin sintered metal. In the case of the oleo-resin, the oil canbe always supplied into the helical screw groove circulation path fromthe oleo-resin, and hence a long-term lubricating function can besecured maintenance-free, and an anti-abrasion property can be enhanced.

[0105] (Second Embodiment)

[0106]FIG. 8A is an explanatory view showing the principle of the ballscrew device in a second embodiment of the present invention. FIG. 8B isa sectional view of the spacer. FIG. 9 is a side view showing the ballscrew device in the second embodiment of the present invention.

[0107] In the second embodiment, as shown in FIG. 8A, supposing that allthe balls and all the spacers 10 inserted into the helical circulationpath defined by the screw grooves 3, 4 be converged on one side, a gapformed between a leading ball (LEAD-B) and a tailing spacer (TAIL-S) istermed a total gap. Given that a spacing (S1) of this total gap islarger than zero (i.e., S1>0) and that one spacer, i.e., the tailingspacer (TAIL-S) be eliminated, the number of the balls 5 and the numberof the spacers 10 are set so that a spacing (S2) of a gap between theleading ball (LEAD-B) and a tailing ball (TAIL-B) is smaller than a0.8-fold value of a diameter (ds, see FIG. 8B) of the spacer (i.e.,S2<0.8×ds).

[0108] The spacings (S1, S2) of the gaps can be controlled by, to bespecific, as shown in FIG. 9, changing design values of a notch height(h) of the circulation tube 14, a rake angle (γ) of the ball 5 and thebend radius (R) of the circulation tube 14.

[0109] As described above, the spacing (S1) of the total gap in thecirculation path is set such as S1>0, and the spacing (S2) of the gapbetween the leading ball (LEAD-B) and the tailing ball (TAIL-B) in thecase of eliminating the single spacer (TAIL-S) is set such as S2<0.8×ds.Therefore, it never happens that the spacer 10 is skewed in thecirculation path because of the intra circulation path spacing being toolarge. There is also no possibility in which the operationaldeterioration is induced due to the friction between the ball 5 and thespacer 10 because of the intra circulation path spacing being too small.The intra circulation path spacings (S1, S2) are properly set, and hencethe spacer 10 is never skewed at approximately 60° or more within thecirculation path, and the good operability can be maintained.

[0110]FIG. 10 is a side view of the ball screw device, showing amodification of the second embodiment of the present invention. In thismodification, there are prepared several types of spacers 10 each havinga different width. For example, as shown in FIG. 10, there are preparedseveral spacers 10 having a width A, several spacers 10 having a widthB, several spacers 10 having a width C, . . . , and the spacings (S1,S2) of the gaps are controlled corresponding to differencestherebetween. In this case too, since the spacings (S1, S2) in thecirculation path are properly set, it never happens that the spacer 10is skewed at approximately 60° or larger in the circulation path, andthe good operability can be maintained. Note that the diameter of thespacer 10 is not changed, and hence there is no necessity for speciallydesigning the nut 2.

[0111] Incidentally, for instance, the sectional configuration of thespacer is not necessarily the Gothic arch but may be applied as a singleradius or a U-shape.

[0112] Further, examples and comparative examples of the secondembodiment will be discussed later on.

[0113] (Third Embodiment)

[0114]FIG. 11 is an enlarged view showing the ball and the spacer of theball screw device in a third embodiment of the present invention. FIG.12 is a side view showing the ball screw device in the third embodimentof the present invention.

[0115] The ball screw device in the third embodiment illustrated FIG. 12is classified as the tube circulation type ball screw device including acirculation tube 14, formed in association with the screw grooves 3, 4,and through which the balls 5 and the spacers 10 are circulated.

[0116] The circulation tube 14 is also formed with a bend radius.According to the third embodiment too, this bend radius (R) is set equalto the radius of the ball center diameter (BCD) of the screw groove 3 ofthe screw shaft 1.

[0117] As shown in FIG. 11, the spacer 11 made from a sphere is formedwith the two concave surfaces 11. The section of each concave surface 11may be formed of two circular arcs of which central positions deviatefrom each other to form a Gothic arch, or it may take otherconfigurations. The spacer 10 is constructed to contact the ball 5 atcontact points indicated by the numeral 20.

[0118] In the third embodiment, the spacer 10 is integrally formed froman elastically deformable material such as a resin etc, and a slit 21 isformed in an outer peripheral surface of the spacer 10. The spacer 10 isthereby elastically deformed due to a flexure of the slit 21 between theballs 5, 5, and contacts the balls 5 at the contact points 20. At thesame time, a spacing (d) between the concave surface 11 of the spacer 10and the outer peripheral surface of the ball 5 can be extended andreduced. Accordingly, a distance (L) between the balls 5, 5 can becontrolled by elastically deforming the spacer 10, and a charging rateof the balls 5 and the spacers 10 with respect to a circuit length caneasily be set to a proper value. For example, the charging rate can becontrolled by one type of the integrally formed spacers, which obviatesa troublesome design work of preparing several types of spacers on atrial basis and combining these spacers. Further, it is also possible toattain a charging rate of 100% (i.e., the spacing between the ball andthe spacer is zero) as the necessity may arise, and the cost thereforedecreases.

[0119] Note that the spacer 10 may be elastically deformed in terms of astructure as in the case of the slit 21 described above, or may also beelastically deformed based on only the material itself as in the case ofthe resin and a rubber etc.

[0120] Moreover, as shown in FIG. 11, the through-hole 12 for receivingthe oil may also be formed between the two concave surfaces 11 of thespacer 10.

[0121]FIG. 13 is an enlarged view of the ball and the spacer of the ballscrew device in a modification of the third embodiment of the presentinvention.

[0122] In this modification, the concave surface 11 of the spacer 11takes slightly a conical shape, and the spacing (d) between the concavesurface 11 of the spacer 10 and the outer peripheral surface of the ball5 is set larger than in the case of FIG. 11.

[0123] Further, the slit 21 of the spacer 10 is formed in a V-shape. Inthis case too, the spacer 10 is elastically deformed based on theflexure of the slit 21 between the balls 5, 5, and is brought intocontact with the balls 5 at the contact points 20. At the same time, thespacing (d) between the concave surface 11 of the spacer 10 and theouter peripheral surface of the ball 5 can be extended and reduced, andhence the proper value of the charging rate of the balls 5 and thespacers 10 with respect to the circuit length can easily be set bycontrolling the distance (L) between the balls 5, 5.

[0124] (Fourth Embodiment)

[0125]FIG. 14 is a perspective view showing a linear guide in a fourthembodiment of the present invention. FIG. 15 is a sectional view of thelinear guide shown in FIG. 14. FIG. 16 is a sectional view showing theballs installed in the linear guide shown in FIG. 14, and the spacerinterposed between the balls.

[0126] As illustrated in FIG. 14, a slider 32 taking a U-shape insection, and which is defined as an outer member, is disposed astride ofa guide rail 31 defined as an inner member assuming substantially arectangular shape in cross-section. As illustrated in FIG. 15, trackgrooves 33 a each taking a configuration of circular arc and extendingin the axial direction, are formed in two right-and-left side surfacesof the guide rail 31.

[0127] Leg members 34 provided on two right-and-left sides of the slider32 are also formed with track grooves 33 b each taking the configurationof a circular arc and extending in the axial direction. A travel path ofthe balls 35 is defined by the track groove 33 a of the guide rail 31and the track groove 33 b of the slider 32.

[0128] Further, hole-like return paths 36 are formed more outward thanthe paths 33 of the two leg members 34 of the slider 32. The paths 33and the return paths 36 communicate with each other via turn-paths 37 atan end portion thereof. The circulation path of the balls 35 is thusconstructed of the travel-paths 33, the return-paths 36 and theturn-paths 37.

[0129] Moreover, as shown in FIG. 16, a spacer 39 formed with twoconcave surfaces 38, 38 facing respectively to the adjacent balls 35,35, is disposed between the balls 35, 35. A curvature (1/R) of theconcave surface 38 is set larger than a curvature (1/r) of the ball 35,whereby the spacer 39 is so structured as to linearly contact theadjacent balls 35, 35 at outer edges or at portions vicinal to the outeredges.

[0130] Accordingly, the spacer 39 is capable of retaining the ball 35 ina much wider area, and it is feasible to take a still larger retainingallowance for the spacer 39 to retain the ball 35. Therefore, the ball35 is easy to stabilize, and a measurement of a dimension (i.e., athickness of the spacer 39) between the balls 35 is facilitated, wherebythe spacer 39 exhibiting a high precision can be manufactured.

[0131]FIG. 17 is a sectional view of the ball and the spacer, showing afirst modification of the fourth embodiment of the present invention.

[0132] In the first modification of the fourth embodiment, the spacer 39assumes such a configuration that central portions 40, 40 on both sidesin section are recessed and rectilinearly connected to the outer edges.With this configuration, the spacer 39 is so constructed as to linearlycontact the adjacent balls 35, 35 at the outer edges or at the portionsvicinal to the outer edges, and the ball 35 is thereby easy tostabilize.

[0133]FIG. 18 is a sectional view of the ball and the spacer, showing asecond modification of the fourth embodiment of the present invention.The spacer 39 formed with the two concave surfaces 38, 38 facingrespectively to the adjacent balls 35, 35, is disposed between the balls35, 35. The section of the concave surface 38 is, as in the discussionon the first embodiment, formed of the two circular arcs of whichcentral positions deviate from each other to form a Gothic arch.

[0134] With this configuration, as in the first embodiment, the ball 35is allowed to contact the concave surface 38 of the spacer 39 with anextremely low friction, thereby making it feasible to reduce the slideresistance between the these balls 35 and the spacer 39 and at the sametime facilitate the stabilization of the balls 35. Consequently, thespacer 39 comes to have a high circulative characteristic, and it ispossible to restrain the decline of the operability, which might becaused by the impingement between the balls 35, 35, and remarkablyrestrain the balls 35 from being damaged by the friction. There is alsono possibility of inducing fluctuations in torque, fluctuations inkinetic friction and the noise problem as well.

[0135]FIGS. 19, 20 and 21 are sectional views of the balls and thespacers, showing third, fourth and fifth modifications of the fourthembodiment of the present invention.

[0136] In the third, fourth and fifth modifications, a through-hole 41is formed in the central portion of each of the spacers 39 in the firstand second modifications of the fourth embodiment. For example, if thethrough-hole 41 contains the lubricating agent such as the lubricatinggrease and the oleo-resin etc, a retainability thereof is enhanced. Thelubricating agent serves to make much smaller the slide resistancebetween the balls 35 and the spacer 39, and the friction therebetweencan thereby be remarkably reduced, and the spacer 39 can exhibit ahigher circulative characteristic.

[0137]FIGS. 22 and 23 are sectional views of the balls and the spacers,showing sixth and seventh modifications of the fourth embodiment of thepresent invention.

[0138] The sixth and seventh modifications have such a construction thatin the first and second modifications described above the outer edges ofthe spacer 39 are chamfered, and the balls 35 come into contact with theportions vicinal to the outer edges of the spacer 39. In this case too,the balls 35 are easy to stabilize. Further, a durability of the spacer39 is improved by restraining an abrasion and a fatigue of the concavesurface of the spacer 39, with which the balls 35 are bought intocontact.

[0139] Incidentally, other than the sixth and seventh modifications, theouter edges of the spacer 39 with which the balls 35 come into contactare each formed in an edge-like shape but may be C-chamfered orR-chamfered, for instance. Furthermore, the spacers in the sixth andseventh modifications of the fourth embodiment are all integrallyformed.

[0140] (Fifth Embodiment)

[0141]FIG. 24A is a sectional view showing the spacer installed in thelinear guide in a fifth embodiment of the present invention. FIG. 24B isa side view of this spacer.

[0142] As illustrated in FIGS. 24A and 24B, in accordance with the fifthembodiment, cross-grooves 42 are formed in two side surfaces of thespacer 39 as shown in FIG. 16, and outer edge portions a, b, c, d aredisposed equally along four corners of the intersection of thecross-groove 42. Accordingly, the balls 35 are capable of contacting theouter edge portions a, b, c, d equally disposed along the four cornersand therefore contacting the spacer 39 with an extremely low friction.It is therefore feasible to enhance the circulative characteristic ofthe balls 35 and of the spacer 39 by reducing the slide resistancebetween the balls 35 and the spacer 39.

[0143] Further, the lubricating agent can be taken in between the spacer39 and the balls 35 through the cross-groove 42, and the slideresistance between the balls 35 and the spacer 39 can be made muchsmaller.

[0144] Note that the fifth embodiment may also be modified in a varietyof forms. For example, the outer edge portions a, b, . . . with whichthe balls 35 are brought into contact, are not necessarily equallydisposed in the four locations but may be disposed in at least three ormore locations. Moreover, the portion with which the ball 35 comes intocontact is not necessarily the outer edge but may be the portion vicinalto the outer edge. Further, if capable of making a contact area as smallas possible and besides stably retaining the balls 35, the concavesurface of the spacer 39 may be provided with the ball contact portionsin any three or more positions. Still further, the edges of the spacer39, with which the balls 35 are bought into contact, are formed in theedge-like shape but may be C-chamfered or R-chamfered.

[0145] (Sixth Embodiment)

[0146]FIG. 25 is a sectional view showing the spacer installed in thelinear guide in a sixth embodiment of the present invention.

[0147] As illustrated in FIG. 25, in the sixth embodiment, the spacer 39is formed with the two concave surfaces 38, 38 to face adjacent balls35, 35, when disposed between the balls 35. The spacer 39 has athrough-hole 41 formed in a thinnest-portion between the two concavesurfaces 38. Accordingly, the contact area of the spacer 39 with theballs 35 is further reduced due to the through-hole 41 of the spacer 39,and it is possible to remarkably decrease the fluctuations both intorque and in kinetic friction. At the same time, since the through-hole41 is formed in the minimum-thickness portion between the concavesurfaces 38, there is an advantage of minimizing an influence upon thestrength thereof.

[0148]FIG. 26 is a sectional view of the spacer installed in the linearguide, showing a modification of the sixth embodiment of the presentinvention.

[0149] In this modification, recesses 43, 43 each taking approximately atrapezoidal shape are formed instead of the concave surfaces 38 in bothside surfaces of the spacer 39. The through hole 41 is formed in athinnest portion of the spacer 39. Accordingly, in this case too, it isfeasible to remarkably decrease both the contact area of the spacer withthe ball 35 and the influence upon the strength thereof.

[0150] Note that the sixth embodiment may also be modified in manyforms.

[0151] Further, examples and comparative examples of the sixthembodiment will be explained later on.

EXAMPLES

[0152] Examples and comparative examples of the second embodimentdiscussed above are carried out in a way which follows.

Examples of Second Embodiment

[0153] By way of an example of the second embodiment, there is preparedthe ball screw device in which the spacer (a retaining piece) having adiameter ds of 5.6 mm is inserted, wherein as shown in Table 1 thecharging rate is set at 99.0%, the spacing (Si) of the above-mentionedtotal gap is set to 3.6 mm, the spacing (S2) of the gap is set to 4.4mm, and a ratio S2/ds is set at 0.79. TABLE 1 Clearance quantity [mm]Operability Classifi- Charging S2 Torque cation Rate [%] (S2/ds) S1 DataFeeling Example Retaining 99.0 4.4  3.6 ⊚ piece (0.79) Comparative Sameas 100.6 0.8 0 or Δ example 1 above (0.14) under Comparative Same as97.3 11.5 10.7 X example 2 above (2.1) (lock) Comparative Conven- 98.5 —— ◯ example 3 tional member (all balls)

[0154]FIG. 27 shows a test result of this example.

[0155] The fluctuation in torque is extremely small, and it is thereforeconfirmed that the operation condition is good.

Comparative Example 1 of Second Embodiment

[0156] By way of a comparative example 1, there is prepared the ballscrew device in which the spacer (the retaining piece) is inserted,wherein as shown in Table 1 the charging rate is set at 100.6%, thespacing (S1) of the above-mentioned total gap is set to 0 or under, thespacing (S2) of the gap is set to 0.8 mm, and the ratio S2/ds is set at0.14. FIG. 28 shows a test result of this comparative example 1. Thetotal gap etc is set too small, and hence the fluctuation in torque islarger than in the case of the above example (FIG. 27), and it isconfirmed that the operation condition is not so good.

Comparative Example 2 of Second Embodiment

[0157] By way of a comparative example 2, there is prepared the ballscrew device in which the spacer (the retaining piece) is inserted,wherein as shown in Table 1 the charging rate is set at 97.3%, thespacing (S1) of the above-mentioned total gap is set to 10.7 mm, thespacing (S2) of the gap is set to 11.5 mm, and the ratio S2/ds is set at2.1.

[0158]FIG. 29 shows a test result of this comparative example 2. Aninitial operation is well done, however, the total gap etc is set toolarge, and hence the good operation condition is unable to be keptduring the stroke, resulting in a locked state.

Comparative Example 3 of Second Embodiment

[0159] By way of a comparative example 3, the ball screw device whichdoes not use the spacer is prepared, as shown in Table 1, the chargingrate is set at 96.5%.

[0160]FIG. 30 shows a test result of the comparative example 3. Thefluctuation in torque is slightly larger than in the case of the aboveexample (FIG. 27), and it is confirmed that the operation condition iscomparatively good but is inferior to the example (FIG. 27).

[0161] Next, examples and comparative examples of the sixth embodimentwill be given.

Examples and Comparative Examples of Sixth Embodiment

[0162] As shown in FIG. 31, there is measured a kinetic frictionalforce, wherein the spacer is formed with the through-hole in theexample. As shown in FIG. 32, there is measured a kinetic frictionalforce, wherein the spacer is not formed with the through-hole in thecomparative example. It is confirmed that the fluctuation in kineticfriction force is far smaller in the example (FIG. 31) than in thecomparative example (FIG. 32).

[0163] Note that some of the spacers in the examples given above havebeen exemplified as having the shape of the Gothic arch in section.However, the sectional configuration is not limited to the Gothic archand may embrace applications of, e.g., a single R-shape and a V-shape aswell.

[0164] As discussed above, in a preferred ball screw device of thepresent invention, the spacer having the two concave surfaces facingrespectively to adjacent balls, is disposed between the balls adjacentto each other, and the section of each of the concave surfaces of thespacer is formed of the two circular arcs of which central positionsdeviate from each other to form a Gothic arch shape. In this case, theload balls are bought into linear- or point-contact with the concavesurfaces of the spacer formed of the circular arcs each taking theGothic arch configuration, and can be therefore circulated along withinthe helical screw grooves while contacting the concave surfaces with anextremely low friction. Accordingly, the friction between the load ballsand the spacer is small, and the spacer comes to have the highcirculative characteristic. It is also feasible to prevent the declineof the operability, which might be caused by the impingement between theballs, and also frictional damage to the balls. Also, the configurationof the spacer can be made smaller than the conventional spacer ball, andconsequently it never happens that the decreases in the load capacityand in the rigidity are induced with a restraint of reducing the numberof the load balls.

[0165] Furthermore, in the ball screw device of the present invention,the total gap in the circulation path is set larger than zero, andbesides, when the single spacer is eliminated, the spacing of the gapbetween the leading ball and the tailing ball is set in the aboverelationship in terms of the numerical values. In this case, the spaceris not skewed within the circulation path because of the gap in thecirculation path being too large, and it never happens that there isinduced the operation deterioration due to the friction between theballs and the spacer because of the gap in the circulation path beingtoo small. The gap in the circulation path is properly set, andtherefore the spacer is not skewed at approximately 60° or larger,whereby the good operability can be maintained.

[0166] Moreover, in the ball screw device of the present invention, thespacer may be so constructed as to be elastically deformable between theballs adjacent to each other, in which case the inter-ball distance canbe controlled by making the spacer elastically deformed. Accordingly,the charging rate of the balls and the spacer with respect to thecircuit length can be extremely easily set to a proper value. Forexample, the charging rate can be controlled by the one type ofintegrally formed less expensive spacers, which eliminates the necessityfor such a complicated design work as to manufacture several types ofspacers for a trial and combine those spacers in many ways. Further, thecharging rate may be set at 100% (i.e., the spacing between the ball andthe spacer is set to zero) as the necessity may arise. Note that thespacer may be elastically deformed in terms of its structure as in thecase of the slit 21 described above, or may also be elastically deformedbased on only the material itself.

[0167] Moreover, in the linear motion device of the present invention,the spacer may take such a configuration that the balls adjacent to eachother come into contact with the outer edges or the portions vicinal tothe outer edges. In this case, the spacer is capable of retaining theballs with much wider areas, and it is feasible to take a still largerretaining allowance for the spacer to retain the balls. Further, theball is easy to stabilize, and the measurement of the dimension (i.e.,the thickness of the spacer) between the balls is facilitated. It istherefore possible to manufacture the spacer exhibiting a high accuracy.

[0168] As discussed above, in the linear motion device of the presentinvention, the spacer is interposed between the balls and has theconcave surfaces with which the adjacent balls are bought into linearcontact. In this case, the friction between the balls and the spacer issmall, and the spacer has the good circulative characteristic. It isalso feasible to prevent the deterioration of the operability, theoccurrence of noises due to the impingement between the balls and adeteriorated quality of sound produced, and also frictional damage tothe balls.

[0169] Moreover, in the linear motion device according to the presentinvention, the balls adjacent to each other may come into contact withat least three or more portions of the spacer, and are therefore capableof contacting the spacer with an extremely small friction. The frictiontherebetween can be minimized by reducing the slide resistance betweenthe balls and the spacer, and the circulative characteristic of thespacer and the balls is enhanced. At the same time, the stabilization ofthe balls is facilitated, and the lubricating agent can be easily ledinto the spacer, thereby making much smaller the slide resistancebetween the balls and the spacer.

[0170] Further, in the linear motion device of the present invention,the spacer has the through-hole formed in the thinnest portion thereof,in which case it is feasible to remarkably decrease the fluctuation inkinetic friction force with the further diminished contact area betweenthe ball and the spacer owing to the through-hole of the spacer. At thesame time, there is an advantage that the influence upon the strength isdecreased because of the through-hole being formed in the thinnestportion between the concave surfaces.

[0171] The present invention has been discussed by way of theembodiments but may be modified in many forms within the range of thegist of the present invention, and these modifications are not excludedfrom the scope of the present invention.

What is claimed is:
 1. A ball screw device comprising: a screw shaft ofwhich an outer peripheral surface is formed with a helical screw groove;a nut of which an inner peripheral surface is formed with a helicalscrew groove corresponding to the helical screw groove of said screwshaft; a helical circulation path defined by the two helical screwgrooves; and; a multiplicity of balls so disposed in said helicalcirculation path as to be capable of rolling, wherein a spacer havingtwo concave surfaces facing respectively to said balls is disposedbetween said balls adjacent to each other, and a section of each of theconcave surfaces of said spacer is formed of two circular arcs taking aGothic arches, of which central positions deviate from each other.
 2. Aball screw device according to claim 1, wherein each of said spacers hassuch a configuration that an outer surface of a portion excluding theconcave surface thereof exists on a surface of a sphere of which acenter is a central point between central positions of said two ballsadjacent to each other, and of which a radius is a distance or shorterfrom the center to said screw groove of said screw shaft.
 3. A ballscrew device according to claim 1, wherein supposing that all said ballsand all said spacers inserted into said helical circulation path beconverged on one side, a gap formed between a leading ball and a tailingspacer is termed a total gap, and given that a spacing (S1) of thistotal gap is larger than zero (S1>0) and that said one spacer, i.e.,said tailing spacer be eliminated, the number of said balls and thenumber of said spacers are set so that a spacing (S2) of a gap betweensaid leading ball and a tailing ball is smaller than a 0.8-fold value ofa diameter (ds) of said spacer (S2<0.8×ds).
 4. A ball screw deviceaccording to claim 1, wherein said spacer is elastically deformablebetween the neighboring balls.
 5. A ball screw device comprising: ascrew shaft of which an outer peripheral surface is formed with ahelical screw groove; a nut of which an inner peripheral surface isformed with a helical screw groove corresponding to the helical screwgroove of said screw shaft; a helical circulation path defined by thetwo helical screw grooves; and; a multiplicity of balls so disposed insaid helical circulation path as to be capable of rolling, wherein aspacer having two concave surfaces facing respectively to said balls isdisposed between said balls adjacent to each other, and supposing thatall said balls and all said spacers inserted into said helicalcirculation path be converged on one side, a gap formed between aleading ball and a tailing spacer is termed a total gap, and given thata spacing (S1) of this total gap is larger than zero (S1>0) and thatsaid one spacer, i.e., said tailing spacer be eliminated, the number ofsaid balls and the number of said spacers are set so that a spacing (S2)of a gap between said leading ball and a tailing ball is smaller than a0.8-fold value of a diameter (ds) of said spacer (S2<0.8×ds).
 6. Alinear motion device comprising: an outer member; an inner member facingto said outer member via a gap; a multiplicity of balls disposed betweensaid outer member and said inner member; and a spacer interposed betweensaid balls, wherein said spacer has such a configuration that said ballsadjacent to each other come into contact with outer edges thereof orportions vicinal to the outer edges.
 7. A linear motion device accordingto claim 6, wherein said spacer is integrally formed and has concavesurfaces with which said balls adjacent to each other come into linearcontact.
 8. A linear motion device according to claim 6, wherein aspacer interposed between said balls, and said balls are brought intocontact with at least three or more portions of said spacer.
 9. A linearmotion device according to claim 7, wherein said spacer is integrallyformed, and has at least three or more portions with which said ballscome into contact.
 10. A linear motion device comprising: an outermember; an inner member facing to said outer member via a gap; amultiplicity of balls disposed between said outer member and said innermember; and a spacer interposed between said balls, wherein said spacerhas a through-hole formed in a thinnest portion thereof.
 11. A linearmotion device comprising: an outer member; an inner member facing saidouter member via a gap; a multiplicity of balls disposed between saidouter member and said inner member; and a plurality of spacers; saidouter member being linearly movable relative to said inner member; eachspacer being disposed between two adjacent balls and having two concavesurfaces facing respectively to said two balls; and said spacers includespacers having different widths such that distances between the ballsseparated by the respective spacers are different depending on saidwidths.
 12. A linear motion device comprising: an outer member; an innermember facing said outer member via a gap; a multiplicity of ballsdisposed between said outer member and said inner member; and pluralforms of spacers differing in width, the number of said forms being lessthan the number of spacers; said outer member being linearly movablerelative to said inner member; each spacer being disposed between twoadjacent balls and having two concave surfaces facing respectively tosaid two balls; and distances between the balls separated by therespective spacers being different depending on said forms of thespacers.
 13. A linear motion device comprising: an outer member; aninner member facing said outer member via a gap; a multiplicity of ballsdisposed between said outer member and said inner member; and aplurality of spacers; said outer member being linearly movable relativeto said inner member; each spacer being disposed between two adjacentballs and having two concave surfaces facing respectively to said twoballs; and an outer periphery of at least one spacer being concave. 14.A linear motion device comprising: an outer member; an inner memberfacing said outer member via a gap; a multiplicity of balls disposedbetween said outer member and said inner member; and a plurality ofspacers; said outer member being linearly movable relative to said innermember; each spacer being disposed between two adjacent balls and havingtwo concave surfaces facing respectively to said two balls; and an outerperiphery of at least one spacer being concave such that the spacer iselastically deformed to be in substantially circular line contact withthe adjacent balls and that a distance therebetween is variable.
 15. Alinear motion device comprising: an outer member; an inner member facingsaid outer member via a gap; a multiplicity of balls disposed betweensaid outer member and said inner member; and a plurality of spacers;said outer member being linearly movable relative to said inner member;each spacer being disposed between two adjacent balls and having twoconcave surfaces facing respectively to said two balls; and an outeredge portion of at least one spacer being chamfered.